Method of and apparatus for regulating the speed of sintering strands



F. CAPPEL METHOD OF AND APPARATUS FOR REGULATING THE SPEED OF SINTERING STRANDS Aug. 9, 1966 Filed Jan. 21, 1963 L R E w W N mik m m V/ m a maxfimmm v M .33 fiawu muxwm F #QBEGQS L 35mm E $$Q M p z llllllllll am wm A Harneys United States Patent 3,265,377 METHGD 0F AND APPARATUS FGR REGULAT- ING THE SPEED OF SINTERING STRANDS Fred Cappel, Frankfurt am Main, Germany, assignor to Dravo Corporation, Pittsburgh, Pa, a corporation of Pennsylvania Filed June 21, 1963, Ser. No. 29%,862 5 Qlairns. (Cl. 263-28) This invention is for a method of and apparatus for regulating the operational speed of a sintering strand such that material charged on the strand is treated throughout a definite length of the strand to achieve a thoroughly processed end product at the discharge point of the strand.

In conventional sintering machines, endless strands carry layered charge material for burning on the strand from an ignition point to a discharge point. The rapidity at which combustion advances downwardly through the layer charged on the sintering strand is dependent on the gas permeability of the layer, which in turn depends on the vacuum or the suction pulled in windboxes stationed beneath the strand and connected to exhaust fans, Air is drawn from above the sintering strand through the charged layer and exhausted through a stack in communi-.

cation with the windboxes. The permeability of the layer also is dependent upon the depth of the layer, the water content of the charge, the granulation and chemical composition of the charged stock and other factors. Also, if the required sintering heat is supplied either wholly or partially by the combustion of admixed solid fuel with the charged ore or the like, then the quantity of the fuel in the mixture is a factor bearing on the advance of combustion down through the charge layer.

With less favorable bed conditions the strand must move slower and with optimum conditions it may be sped up. The rate of travel of a sinter strand must in all cases be kept at a speed such that the charge is thorough 1y treated by the time of its arrival at the discharge end of the sinter strand and at the same time secure maximum output. It is customary in the art to select the rate of travel of the strand so that, for a given thickness of the layer, sufficient time is allowed for the thorough treatment of the material on the strand. However, a speed is normally selected so as to provide a more than adequate overtravel and thereby provide a substantial safety 7 factor in order to assure complete firing of the charge at all times regardless of such variations in the charge as may be encountered. According to this procedure, the sinter strand area is not utilized fully because under all normal conditions the product has been completely fired before reaching the discharge end of the sinter strand.

It has been suggested that the sinter strand speed be regulated automatically by sensing a number of process variables to control the output of the drive motor of the strand. In all events, it is desired to keep the speed as high as possible, and, at the same time, to complete the treatment of the charged stock by the time it reaches the discharge end of the strand. Control variables for regulating a sinter strand speed which have been contemplated are the local waste gas temperature at preselected points along the sinter strand and the chemical composition of the waste gas at such points, with these variables being utilized as indicators of the completion of the sintering or burning of the stock. Although these suggestions have merit, the instrumentation required and the time for analyzing the variables have made such practice unfeasible from both practical and economical standpoints.

It is an object of the present invention to provide a "ice new and improved method and apparatus for the control of the speed of a sintering strand.

Another object is to provide a method and apparatus wherein a control variable of the process is readily sensed and the speed of the sintering strand is regulated accordingly.

It is a further object of this invention to provide a method and apparatus for regulating the speed of a sintering strand wherein only one control variable is sensed for the accurate governing of the sinter strand speed with the selected control variable being extremely reliable.

A complete understanding of the instant invention may be had from the following detailed description of a specific embodiment thereof when read in conjunction with the appended drawing wherein FIG. 1 is a schematic of a sintering machine with the associated control elements for the regulation of the speed of the sintering strand and FIG. 2 is a partial plan view of the sintering machine strand in relation thereto of the air velocity sensing device.

This invention is based on the discovery that to completely burn or sinter a given charge of material, a definite volume of air must pass through the bed, regardless of whet-her it permeates through the bed rapidly or with difficulty. If the flow of air through the bed is free, the volume will pass through in a shorter time than if the bed is dense. Hence, it is possible to utilize the speed of the air current through the bed as a single control factor for regulating the speed of travel of the sintering machine. For example, the volume of air required for the sintering of iron ores, cement raw mix, the burning of lime, dolomite, and the like, amounts to approximately 24,000 cubic feet per net ton of the sinter mix. The volume of air for the sintering of sulfidelead ores is approximately 9,000 cubic feet per net ton of sinter mix, and for the sintering of sulfide zinc ores about 15,500 per net ton of the sinter mixture.

It has been found that for a given charge mixture with constant depth of layer and constant vacuum in the windboxes, the air velocity measured above the central axis or longitudinal centerline of the sinter strand also remains constant throughout the entire length of the burning zone of the sinter strand. Only in the vicinity of the sidewalls adjacent the strand up to maximum distance of approximately sixteen inches from the edge toward the central axis does the gas velocity increase in the direction of the discharge end of the sinter strand, and increasing- 1y so toward the edge. Where a cooling zone is employed beyond the combustion zone of the strand the air velocity increases above the strand central axis as compared to the velocity prevailing in the combustion zone above such axis. Thus, it has been determined that two constants are evident throughout a given sintering process: (1) the velocity of air passed through the central area of the charge layer in the burning zone of the system, and (2) the total air volume passed through the charge layer from the beginning to the end of the process.

Referring now to the figures, the sinter machine is indicated generally by numeral 11. The machine includes a conventional endless sinter strand 12, beneath the upper stretch of which are positioned typical windboxes 13 which are connected with a suction fan 14 for drawing air downwardly through a charge layer 15 on the upper stretch of the strand. Stock is fed onto the strand through a feeder device 16, such as a hopper or the like, at one end of the sinter strand and the layer is carried under the igniting furnace hood 17 where combustion is initiated in the top of the charge of material on the strand. The strand is continuously driven in the usual manner by a motor 23. According to this invention the only test or measured variable utilized for regulating the rate of travel of the sinter strand is the gas velocity. This is measured just above the surface of the charge layer by a sensing device 18, such as a fan-Wheel or hot-wire anemometer, placed above the centnal axis or longitudinal centerline of the upper reach of the strand 12. The device 18 is positioned within the combustion zone 19 of the machine 11 just following the hood 17. The sensed air velocity is transformed to an electrical signal by suitable electrical means and transmitted via line 21 to a suitable controller 22 with a set point voltage manually set therein by suitable instruments connected to line 20. The air velocity, now represented as a voltage value, for example, is compared with the set point voltage and the output signal, which is representative of the difference between the actual air velocity and the predetermined set point, effects an increase or decrease in the speed of motor 23. Thus the rate of travel of the sinter strand 12 is variably adjusted as variation in the permeability of the bed occurs, and thi adjustment takes place early in the firing stage so that there is no lag that is detrimental as occurs where sensing must take place near the end of the burning zone.

The air volume required for the sintering of the charge, as hereinbefore explained, is constant as is the length of the strand for any sintering plant machine. These constants are involved in the computation of the set point. The values of the depth of the bed to be carried on the strand and the bulk density of the charge mixture in net tons per cubic foot are the predetermined variables which are initially involved in deriving the set point value for the controller 22. A these latter pre-determined variable factors change, the revised set point is established in the controller 22 by manual setting. Thereafter when the gas permeability of the charge or the suction in the windboxes then fluctuate during the processing of a predetermined charge of a given bulk density, the air velocity sensed by the device 18 would also change, resulting in a change of speed of the strand through the operation of the controller. It is understood, of course, that the negative pressure in each of the windboxes 13 is maintained constant by adjustment of conventional dampers, not shown. Accordingly, the total volume of air passing through the bed per ton of charge material is held constant to ensure that treatment of the charge is completed at the discharge point of the strand with little or no allowance for overtravel and the full capacity of the strand is utilized.

In order to achieve maximum efi iciency, a set point is selected such that the material has been theoretically completely burned through at a time when it reaches the discharge end point of a sintering strand.

In this case the set point is established according to formula v=1/A L/hs c, in which v represents the rate of travel of the sinter strand in feet per minute, L the length of the sinter band in feet, h the depth of layer in feet, s the bulk density of the charge in pounds per cubic foot/2,000 and c the air velocity in feet per minute. A is the aforementioned part of the set point formula which has been determined for iron ores, cement, lime and the like to be 24,000 cubic feet of air per net ton, for lead ores to be 9,000 cubic feet of air per net ton and for zinc ores to be 15,500 cubic feet of air per net ton.

If a cooling one is desired following the burning zone, the value for A is raised accordingly, that is, to 26,000 as against 24,000 cubic feet per net ton of charge. However, if complete sintering through the charge layer is to be intentionally not accomplished, which in special cases may be desirable, as for example to gain adequate material for recycle as a bed layer on subsequent firings, the value of A is reduced accordingly, for instance 4 from 24,000 to 22,000 cubic feet per net ton of stock.

Aside from its simplicity and reversibility, a special advantage of the method according to the invention con sists of the fact that the test :or measuring impulse can be taken in the beginning phase of the process, practically directly following the location of the ignition hood, so that lapse of time before strand speed is adjusted is practically eliminated.

If the gas velocity above the sinter strand changes because of a changed composition of the charging stock, then in order to achieve maximum efliciency, not only is the rate of travel of the sinter strand changed, but also the quantity or rate stock is fed by the device 16 must be changed in proportion to the gas velocity. These factors are considered in the computation of a set point for the controller according to the formula given. As is well known, in operation of sintering plants, the rate of feed of stock or depth of stock on the sinter strand and the composition of the stock fed to the strand are normally changed only infrequently.

What is claimed is: p

1. An apparatus for regulating the speed of a motor driven sintering strand of a machine having a combustion zone over which a bed of material is carried on the strand for treatment comprising an air velocity sensing device positioned above the bed along the central axis of the strand, means for transforming the air velocity into an electrical signal, a controller having a set point voltage derived by the formula in which v represents the rate of travel of the sinter strand (in feet per minute), L the length of the sinter band (in feet), It the depth of layer (in feet), s the bulk density of the charge (in pounds per cubic foot/2,000), c the air velocity (in feet per minute), and A is a constant (in standard cubic feet per net ton of charge) dependent on the type of raw material to be treated on the strand, the output of the controller being an error voltage which is the resultant of the comparison of the voltage signal from the air velocity sensing device with the set point voltage, and means connected to the output of the controller and to the drive motor of the strand for varying the speed of the drive motor in proportion to the output error voltage.

2. An apparatus for regulating the speed of a motor driven sintering strand as in claim 1 wherein the air velocity sensing device is a fan Wheel.

3. An apparatus for regulating the speed of a motor driven sintering strand as in claim 1 wherein the sensing device is a hot-wire anemometer.

4. A method for regulating the rate of burning of a uniformly thick layer of a predetermined mixture of material upon a porous moving sintering strand solely by the speed of travel of 'said strand, comprising the steps of,

(a) uniformly feeding a layer of a predetermined mixture of materials having a known bulk density to the front end of the moving strand,

(b) igniting the surface of said moving layer,

(c) uniformly feeding air at a preselected velocity to the surface of said strand layer from the point of ignition to the discharge end of the strand,

(d) continuously measuring the velocity of the air moving through the layer of mixture upon the strand at the longitudinal centerline thereof immediately following said ignition of the surface of the layer, and

(e) regulating the speed of movement of the strand in accordance with change in the measured permeability of the burning mixture at the said longitudinal centerline of the strand.

5. The method as defined in claim 4 wherein the speed of the strand travel is determined for any preselected mixture of materials as, V: l/A L/hs C wherein 5 6 V=strand speed in feet/min. References Cited by the Examiner L=1ength of strand, in feet, from loadpoint to discharge UNITED STATES PATENTS point. h=depth of the layer of material in feet, upon the St an 2,410,944 11/ 1946 Johnson 266-21 s=bulk density (pounds per cu. foot/2000). 5 3,050,299 8/1962 Reed 266-25 X C=air velocity (feet/min). 3,138,014 6/1964 Jorre 26621 X A=a predetermined constant (cu. ft. of air/net ton of mixture charged to strand) a 24,000 s.c.f. for iron FREDERICK L. MATTESON, JR., Primary Examiner.

ore, raw cement mix, lime and dolomite; 15,500 s.c.f. for zinc ores and 9,000 s.c.f.; 9,000 for lead ores. 10 JOHN CAMBY Asslstam Examiner 

4. A METHOD FOR REGULATING THE RATE OF BURNING OF A UNIFORMLY THICK LAYER OF A PREDETERMINED MIXTURE OF MATERIAL UPON A POROUS MOVING SINTERING STRAND SOLELY BY THE SPEED OF TRAVEL OF SAID STRAND, COMPRISING THE STEPS OF, (A) UNIFORMLY FEEDING A LAYER OF A PREDETERMINED MIXTURE OF MATERIALS HAVING A KNOWN BULK DENSITY TO THE FRONT END OF THE MOVING STRAND, (B) IGNITING THE SURFACE OF SAID MOVING LAYER, (C) UNIFORMLY FEEDING AIR AT A PRESELECTED VELOCITY TO THE SURFACE OF SAID STRAND LAYER FROM THE POINT OF IGNITION TO THE DISCHARGE END OF THE STRAND, (D) CONTINUOUSLY MEASURING THE VELOCITY OF THE AIR MOVING THROUGH THE LAYER OF MIXTURE UPON THE STRAND AT THE LONGITUDINAL CENTERLINE THEREOF IMMEDIATELY FOLLOWING SAID IGNITION OF THE SURFACE OF THE LAYER, END (E) REGULATING THE SPEED OF MOVEMENT OF THE STRAND IN ACCORDANCE WITH THE CHANGE IN THE MEASURED PERMEABILITY OF THE BURNING MIXTURE AT THE SAID LONGITUNDINAL CENTERLINE OF THE STRAND. 